The present invention relates to a method for the preparation of an organopolysiloxane having a ladder-like molecular structure. More particularly, the invention relates to an efficient and reproducible method for the preparation of an organopolysiloxane having a ladder-like molecular structure of a high degree of polymerization having a relatively narrow molecular weight distribution and useful as a resist material, coating material and the like.
An organopolysiloxane having a ladder-like molecular structure, referred to as a ladder polysiloxane hereinbelow, is a known polymer in the prior art and represented by the general formula ##STR2## in which R is a substituted or unsubstituted monovalent hydrocarbon group having 1 to 20 carbon atoms and the subscript n is a positive integer. Among several known methods for the preparation of such a ladder polysiloxane, disclosures in Japanese Patent Publications No. 37-550 and No. 40-15989 teach a method in which an oligomeric polysiloxane having a cage-like or double-ring structure formed by the dehydration condensation for ring-formation between the silanolic hydroxy groups in each of the upper and lower polysiloxane sequences in the general formula (I), in which the value of n is, for example, 4 to 12, is admixed with an alkali catalyst such as potassium hydroxide and subjected to thermal polymerization in a solution. Although a ladder polysiloxane having a relatively narrow molecular weight distribution can be obtained by this method when the preparation is conducted in a small scale, this method is not always quite satisfactory when conducted in a large scale in respect of the homogeneity of the polymerization reaction and the broad molecular weight distribution of the product because, due to the very limited amount of the solvent usable in the polymerization mixture, the polymerization reaction proceeds in a seemingly solid phase so that uniformity in the temperature of the polymerization mixture can hardly be ensured sometimes to cause gelation of the polymerization mixture at or in the vicinity of the reactor walls leaving, on the other hand, the core portion of the polymerization mixture remote from the reactor walls polymerized only insufficiently.
An alternative method is disclosed in Japanese Patent Kokai No. 57-18729, according to which a ladder polysiloxane of a relatively narrow molecular weight distribution can be obtained by using a carbodiimide compound as the catalyst. To the contrary to the disclosure, the reproducibility of this method is not high enough so that the molecular weight distribution of the product is sometimes too broad in addition to the disadvantage that a ladder polysiloxane having a high molecular weight can hardly be obtained by this method.
Further alternatively, a method is disclosed in Japanese Patent Kokai No. 59-108033 by using a fluoride of an alkali metal as the catalyst. This method is advantageous in respect of the increased reaction velocity as compared with the method by using an alkali catalyst but the problem of the inhomogeneous reaction still remains unsolved because the polymerization reaction in this method also proceeds in a seemingly solid phase resulting in a broad molecular weight distribution of the polymer product.